Proteinaceous materials are used for a wide variety of applications. Predominant useful properties are their ability to swell in aqueous solutions and yet form a solid matrix which is permeable to aqueous solutions upon drying. These properties have been exploited for many generations in the field of photographic sciences and proteinaceous materials are still widely used as a binder for harbouring silver halide grains in the photosensitive layer of photographic films. A particular type of proteinaceous material is gelatin as commonly employed in gelatin coatings.
Gelatin coatings are used in various fields of technology, for example, as protective coatings on objects or as binder coatings for reagents in materials for analytical or diagnostic purposes, or for light-sensitive materials, preferably silver halides, in photographic recording materials. For practical use, these coatings are hardened by the addition of a hardener. Known hardeners act by cross-linking the gelatin as a result of a reaction with its free amino, imino, or hydroxyl groups.
Formation of a solid matrix is typically considered to be a result of inter-and intra-molecular hydrogen bonding within both the helical and random regions of proteinaceous materials. If only the natural hydrogen bonding is employed the strength of the matrix is typically insufficient for use in a photographic film. Therefore, it is common practice to add a hardener, also known as a crosslinking agent, to a proteinaceous material when used for photographic layers.
For this purpose, it is desirable that the hardening reaction be complete shortly after layer formation. Thus, the coatings attain their complete functionality, based on hardening, immediately after manufacture. Particularly in photographic recording materials, this avoids, for a long time after manufacture, changes in photographic properties as a result of the so-called "afterhardening".
Hardeners are chosen, in part, for their ability to link one group on a proteinaceous molecule with another group on the same, or different, proteinaceous molecule. The linking generates a three dimensional network of proteinaceous material. This three-dimensional network has sufficient strength to safely harbour a silver halide grain. Another important aspect of the three dimensional network is an ability to allow solution to permeate freely during the photographic processing steps of development, fixing (or bleaching) and washing. It is imperative that the solution which freely permeates the matrix is not strongly absorbed. This is particularly important for photosensitive elements since they must often be capable of transiting the photographic processing steps of development, fixing, washing and drying in 20-120 seconds.
Crosslinking of a binder matrix most often involves the carboxyl groups, amine groups, or combinations thereof. The number of carboxyl groups is substantially larger than the number of amine groups in most commercially available gelatin. Traditional hardeners, such as triazines, are widely accepted as capable of combining amine groups and are thus termed amine-amine crosslinkers. Amine-amine crosslinkers provide a very strong matrix yet the carboxyls are largely unaffected. The unreacted carboxyl groups are deleterious since they strongly absorb processing solutions and increase the time required to remove the absorbed solution. The result is an increase in the time and/or energy required for transiting the photographic processing steps identified above. Peptide couplers, such as imidazoliums, are widely accepted as combining a carboxyl group with an amine group to form an amide linkage between binder strands. This is advantageous since the number of free carboxyls is decreased. Unfortunantly, the strength of the peptide-coupled binder is insufficient to transit a processor and total binder destruction is frequently observed.
Peptide couplers generally react very quickly relative to amine-amine crosslinkers. Therefore, the availability of free amine groups is expected to be diminished after peptide coupling. The diminished number of available amine groups is expected to decrease the effectiveness of amine-amine crosslinking and therefore the two methods of crosslinking are considered in the art to be competitive as opposed to complementary.
There has been a long felt need in the art to provide a method of crosslinking a binder which has the strength of an amine-amine crosslinked matrix and the permeability and low solution retention of a binder crosslinked by amide linkages.
So-called instant hardeners are described, for example, in Reif, U.S. Pat. No. 5,034,249; Liebe, German Patent 3,819,082 (European Patent Application 345,514); Himmelmann, U.S. Pat. No. 4,063,952 (German Patent 2,439,551); and Himmelmann, U.S. Pat. No. 3,880,665 (German Patents 2,317,677 and 2,225,230). In these hardeners, a carbamoyl group doubly substituted on the nitrogen is linked to a quaternized nitrogen atom of a heterocyclic ring, generally a pyridine ring. Imidazolium rings are not disclosed and the rings described in the prior art are inferior to imidazolium for the reasons set forth herein.
However, the known instant hardeners are only slightly stable in aqueous solution. It is therefore not possible to prepare a supply of aqueous solutions of these compounds for practical use, because the hardening effect decreases with time as the content of the reactive material decreases in storage. In addition, solutions and coatings prepared with pyridinium hardeners have an unpleasant pyridine odor in use.
An important consideration in crosslinking a binder is the pH of activity. This is particularly important when comparing amine-amine crosslinking reactions with reactions that form amide linkages. Amine-amine crosslinkers, like triazines, are typically stable around a neutral pH (.about.7) and decomposition, or decreased reactivity, is observed above or below neutrality. Peptide couplers, especially imidazoliums, are susceptible to instablility at higher pH and decomposition is accelerated above a pH of approximately 6.2. Therefore, if a pH is employed for optimum amine-amine crosslinking, the decomposition of imidazolium complexes is in competition with crosslinking reactivity. If a pH is employed which is suitable for formation of amide linkages by an imidazolium, the amine-amine crosslinking reagents become unstable in solution. At intermediate ranges neither crosslinking method is efficient. Partly due to the stability differences, skilled artisans have considered imidazolium type coupling agents and amine-amine coupling agents to be incompatable since a suitable solution pH was unavailable. By methods described herein the advantages of imidazolium couplers and amine-amine couplers can be used concurrently to provide a strong matrix with low water absorption.
Therefore, the problem involved in the invention is to provide instant hardeners that are stable in aqueous solution and yield odor-free, hardened coatings in practice.